Field of the Art
This invention relates to a process for production of foamed or cellular concrete molded articles in a so-called prefoam manner by subjecting a molded article obtained from the slurry of a rapid-hardening cement compound to high-temperature and high-pressure curing. More particularly, the invention relates to a process for producing cellular concrete which is characterized by the method of controlling the setting of a foamed slurry of the rapid-hardening cement compound and of developing the strength of the molded article to be subjected to high-temperature and high-pressure curing.
Cellular concrete has been spotlighted from the view points of weight reduction and energy saving in concrete construction. The so-called prefabrication of concrete products has been developed, and a variety of cellular concrete panels have been used.
Such cellular concrete is represented by autoclaved light-weight concrete (hereinafter referred to as ALC). ALC has a large market as stable building material in various forms and is a light-weight concrete (LC) comprised of crystalline calcium silicate hydrate of tobermorite species which has been produced by curing prefoamed set products in an autoclave under pressurized hydrothermal conditions.
The ALC now produced on an industrial scale can be classified into the so-called post-foam type and the pre-foam type in view of the difference in forming cellular structures. The former, post-foam process, is carried out by introducing a hydraulic cement compound slurry into a mold and foaming it with hydrogen gas generated from the reaction of aluminum powder added into the cement compound with an alkali component which comes from cement or lime. According to this process, the height of the foamed mass in a mold is limited to about 60 cm in order to secure uniformity of upright direction foaming conditions, in view of the static pressure of the cement compound slurry under foaming in the mold. Thus, in order to increase the efficiency in the use of costly molds, panels or slabs (before curing) of 60-cm width, due to the above mentioned foamed height, the resulting foamed products are produced by slicing vertically to a predetermined thickness.
As is clear from the above described procedure, conventional cellular concrete panels of the post-foam type are not satisfactory for large-sized products, complicated shapes, designs, etc.
Such problems can be essentially solved by the so-called pre-foam process wherein foam is introduced into a hydraulic cement compound slurry, and then the foamed slurry is poured into a mold. In this case, however, one mold is used for each panel or slab. Therefore, it is essential to shorten the time between pouring and demolding by rapidly hardening the cement compound slurry in order to increase the efficiency in the use of the costly molds. On the other hand, in order to (i) prevent such phenomena as local setting of the cement slurry, segregation of high-density components, and defoaming in the mold in the procedure between pouring and demolding, and to (ii) carry out surface treatment such as scraping (leveling) of protuberant portions over the mold, planing of the surface, or stamping design patterns, it is essential that the cement slurry under setting have suitable consistency for a period necessary for such surface treatment. In view of this necessity, one measure which may appear to be appropriate is to incorporate a setting retarder in the rapid-hardening cement compound slurry. However, it is not always easy to select a setting retarder fit for the purpose.
On the other hand, when a calcium aluminate is used as a rapid-hardening cement compound, it is necessary to pay attention to its transition reaction after hardening. That is, calcium aluminates are usually derived from minerals such as C.sub.12 A.sub.7, CA, CA.sub.2, C.sub.3 A, C.sub.4 AF, and C.sub.3 A.sub.3 CaSO.sub.4 (wherein C denotes CaO and A denotes Al.sub.2 O.sub.3). These produce minerals having a large amount of water of crystallization at the begining of the hydration reaction and are gradually converted into minerals having less water of crystalization after heating. For example, a representative reaction for the hydration of CA is as follows. EQU CAH.sub.10 .fwdarw.C.sub.2 AH.sub.8 .fwdarw.C.sub.3 AH.sub.6
This reaction is a transition reaction. It is known that, especially in the process of C.sub.2 AH.sub.8 .fwdarw.C.sub.3 AH.sub.6, a decrease in the strength properties of the resulting structure takes place because of the porosity thereof caused by a change in volume or release of water during the reaction. As in the case of the cellular concrete molded articles prior to autoclave curing, the strength of the molded article is still low, and yet the molded article must stand without a support after it has been demolded. Because of the above mentioned decrease in strength during the transition reaction, the molded article may fail to withstand its own weight. Thus, a flow of the structure often occurs and sometimes its collapse.